Shortly after the Hubble Space Telescope was deployed in 1990, the observatory's primary mirror was discovered to have an aberration that affected the clarity of the telescope's early images. Fortunately, Hubble, orbiting 353 miles (569 km) above the surface of the Earth, was the first telescope designed to be visited in space by astronauts to perform repairs, replace parts, and update its technology with new instruments. Astronauts repaired Hubble in December 1993. Including that trip, there have been five astronaut servicing missions to Hubble.